Microsoft has acknowledged a frustrating new issue affecting users of the “new Outlook” for Windows, where Excel attachments fail to open if their filenames contain non-ASCII characters.
The technical glitch, tracked under the reference ID EX1189359, triggers a vague error message advising users to “Try opening the file again later,” leaving many confused about the cause of the blockage.
The problem targets the modern “new Outlook” client explicitly and does not appear to impact the classic version of the software. According to Microsoft’s service health dashboard, the root cause lies in an encoding error within the request used to open the files.
When an Excel file attached to an email includes non-standard characters such as accented letters, symbols, or non-English scripts, the application fails to process the filename correctly, resulting in an immediate error.
This issue has been ongoing since late November 2025, with initial reports surfacing around November 23. While the scope is limited to specific file naming conventions, the impact is significant for international users or organizations that frequently use non-ASCII characters in document titles.
Microsoft’s Response and Fix Status
As of the latest update on December 1, 2025, Microsoft engineers have successfully developed a fix to address the missing encoding in file-handling requests.
However, the solution is not yet available to all users. The company is currently in the validation phase, testing the deployment to ensure it resolves the error without introducing secondary issues.
Microsoft is also investigating why this encoding error occurred in the first place to prevent similar regressions in future updates. A further status update is expected by the evening of December 1, UTC time.
Until the patch is fully rolled out, Microsoft has provided two official workarounds for users who need immediate access to their spreadsheets:
- Use Outlook on the Web (OWA): The web-based version of Outlook correctly processes these attachments, bypassing the client-side encoding failure.
- Download the File Locally: Users can save the attachment to their computer first. Once the file is saved to a local drive, it can be opened directly in Excel without triggering the Outlook previewer error.
This incident joins a growing list of pains for the new Outlook client, which has faced scrutiny from power users for feature-parity gaps compared to the classic COM-based application. Admin administrators can track the progress of the fix in the Microsoft 365 admin center under EX1189359.
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